Electrostatic image copier having photoconductive element switching



"Jan-31,1967 ii-O E 3,301,947

ELECTROSTATIC- I'MAGE'coPIER HAVING PHOTOGONDUCTIVE ELEMENT SWITCHING Filed. April 1, 1963 PAPER P12ocsas-m gv 52 1 TAT VIDEO 6 RAmo $16NAL FREQUENCY h AMPLHHER spewALs PHOTO can.

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Patented Jan. 31, 1967 s 301 947 ELECTROSTATIC IMAGE CoPIEn HAVING PHO- TOCONDUCTIVE ELEMENT SWITCHING Joseph J. Stone, Glenview, IlL, assignor to A. B. Dick Company, Chicago, 111., a corporation of Illinois Filed Apr. 1, 1963, Ser. No. 269,671 5 Claims. (Cl. 178-66) electrostatic charge patterns on paper. The paper is then developedusing a suitable powder which adheres to the charged areas and is thereafter fixed thereto. There are various different types of transducers employed ranging from a selenium plate used in the well known Xerographic 7 process to apparatus which includes photoresistive material which is in a current path whereby charge deposition may be eifectuated.

In all of these arrangements the response of the electrostatic transducer usually determines the end result, and not much can be done to compensate for variables, such as variable linearity, background color changes on the document which the data is written, or poor contrast problems. Nor can much be done for altering color sensitivity or selectivity of these prior art electrostatic transducers.

An object of this invention is the provision of an electrostatic writing system which permits compensation for the characteristics of the electrostatic writing transducer.

Yet another object of the present invention is the pro- 1 vision of the novel electrostatic writing system wherein it is possible to obtain a desired response within the writing system.

Still another object of the present invention is the proyision of a novel, useful and inexpensive electrostatic writing system.

These and other objects of the invention may be achieved in an electrostatic writing system of the type wherein the electrostatic transducer effectively comprises 7 a plurality of separate photoconductive cells which are in the current path for depositing electrostatic charges on the writing medium. Instead of permitting light reflected from the image to fall on these photocouductive cells whereby a pattern charge is deposited on the writing medium, light from a constant source of illumination is permitted to successively fall upon each one of the photoconductive cells or elements while the amplitude of the biasing current source is varied in response to light from the documentwhich is being synchronously scanned. Thus, the constant light source serves to turn on each photoresistive element, much in the manner of a switch being closed whereby a current can flow therethrough hav ing an amplitude determined by the light reflected from the picture element on the document being scanned at the time. The electrostatic charge'deposition which occurs is a function of the amount of current flowing.

Because of this manner of control, it is possible to use many well known electrical circuit techniques to improve contrast response as well as to determine linearity, to obtain any other desired characteristic for the electrical current in response tothe exciting voltage derived from a single scanning photocell. Further, response to differ ent colors is much easier to accomplish either selectively ductive as is possible.

or for a wide band since a much better and much wider range of characteristics is achievable in photoelectric cells rather than in the usual solid state photocell. Also, one may obtain a negative instead of a positive by simple circuit techniques.

The'novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is an isometric view of an embodiment of the invention;

FIGURE 2 is a cross section of an electrostatic transducer of a type suitable for use with this invention; and

FIGURE 3 is another embodiment in accordance with this invention.

This invention will be described employing an electrostatic light image transducer of a type which is described and claimed in an application by Joseph J. Stone and John H. Borland for a Light Image Converter, filed April 10, 1962, having Serial No. 186,474, now US. Patent No. 3,186,839 and being assigned to this assignee. How'- ever, this is by way of exemplification and is not to b construed as a limitation upon this invention.

FIGURE 1 is an isometric view of an embodiment of this invention. A document 10, which is to be scanned is mounted upon a drum 12, in any suitable manner. Adjacent the location of the document 10 and extending around the drum surface is a translucent strip 14. Mounted within the drum, as represented by the dotted lines, is a light source 16 which shines light through a small spot designated as 16A, through the translucent ring area. A lens 18, focuses a line of the image on the document 10 as well as the spot of light adjacent thereto upon a rotating many sided mirror 20. A drive source 22 drives the drum and mirror synchronously with one another.

As the mirror rotates, it serves to pass a spot of light followed by the line of data across a photocell 24, and across an electrostatic image transducer 26, positioned adjacent thereto. As will be seen in FIGURE 2, the image transducer 26, comprises a plurality of photoresistive cells extending across the image converter portion 26A,- of the electrostatic transducer. Spaced from the back surface of the image converter, is a conductive back bar 26B. Paper of any other suitable medium 28, upon which it is desired to write, passes between the image converter 26A and the back bar 268 and receives an electrostatic charge deposition thereon.

The mirror is rotated in a manner so that it will successively reflect successive lines from the document on the drum past the cells on the image converter. The spot of light is made sufficiently bright or intense to render each one of these cells upon which it falls as highly con- The output of the photocell 24 is applied to a video signal amplifier 30. The video signal amplifier output is applied to the image transducer 26A. A radio frequency signal source 32, has its output connected -to the back bar 26B.

Referring now to FIGURE 2, there may be seen a cross section of the image transducer 26. This includes a glass or other suitable insulator 34, through which a plurality of tiny holes have been drilled. A wire 36A, 36B, 36C, for example, is inserted in each one of these holes and then is finished ofl flush with the surface of the dielectric 34. Thereafter, a portion of the wire is etched away from one surface of the substrate. The etched away region is then filled with a photoconductive material 38A,

38B, 38C, such as cadmium sulphide. A translucent conlayer 40. The radio frequency signal source 32, is connected to the back bar. It has been found that a radio frequency signal when applied to the back bar, reduces the video signal levels required and linearizes the response of the system from the video level to the recorded electrostatic image.

It should now be appreciated, that as each one of the photoconductive elements in the image converter 26A is rendered highly conductive in response to the light from the spot 16A being made to shine thereon, the photocell 24 provides an output signal in response to a picture element which is made to shine thereon. The output signal is applied to the video signal and amplifier 30 which applies a current through the photoconductive element hav- The wire ends ing an amplitude determined by the amplitude of the pic- I ture element which is scanned at the time, and thereby the electrostatic charge deposited by the stylus on the paper 28 corresponds to the intensity of illumination of the picture element. The paper 28, with the charge pattern thereon, representativeof the line of elements which have been scanned, is moved under a paper processing station 42, which serves the function of developing the electrostatic image in accordance with well known techniques.

FIGURE 3 shows another arrangement for the embodiment of the invention which is simpler in construction.

' Both the gear box 58 and the mirror 52 are synchronously driven from a drive source 610. An image transducer 62 is identical with theima-ge transducer 26A and is connected to receive the output of a video signal amplifier 64. The video signal amplifier is driven from the output of the photoelectric cell 54. A radio frequency signal source 66 applies its output to a back bar 68, as before.

I As the many sided mirror 52, rotates, it successively refleets a column of the light variations from the document 10 past the photoelectric cell 54. Simultaneously, the-rer with, light from the source 50 is caused to traverse the image converter 62. The operation of the. arrangement for depositing an electrostatic charge upon the paper 70 is identical with that described in connection with FIG- URE. 1. As the mirror successively rotates, the photoelectric cell 54 is caused to move linearly along the edge of the document 10 whereby the successive columns of picture elements upon the document may be passed by the photoelectric cell. These successive columns of picture elements are recorded as electrostatic charge patterns on the paper 70.

Since the same mirror projector system projects both the image and the light spot, exact synchronization is maintained in the translation of the image past the photocell and the translation of the gating spot along the image bar. The video signal produced by the photocell is caused to produce an image through the styli illuminated by the flying spot. The other styli are not illuminated and therefore nonres-ponsive to the variations of the applied video signal.

Since the signal applied to the image converter is amplified, many things may be accomplished with'it, such as providing a simple mechanism for achieving the positive or a negative image by switching in or removing inverting stages in the amplifier. The gain may be adjusted to provide for better linearity thereby more linear results are achieved. It high contrast copying is desired, then clipping may be introduced, whereby signals below a certain level produce only white images and signals above the predetermined level produce dark images. Because of the wide spectral response achieveable with photoelectric cells as contrasted to the limited spectral response of photoconductive materials, this system of operating an electrostatic transducer can be made to respond to an extremely wide band range of colors as well as to a very narrow spectral range.

There has accordingly been described and shown herein a novel, useful and improved electrostatic image transducing system. i

I claim:

1. In a system for converting a light image to a charge pattern on a writing medium, employing an electrostatic image transducer of the type comprising a plurality of photoconductive elements each of which is in the electrical curent path employed for depositing electrostatic charges on said medium representative of light image variations, the improvement comprising a source of electrical current, a single photocell connecting said source of electrical current to all of said photoconductive elements, a source of light having a fixed level of illumination, and means for successively illuminating said photoconductive elements with light from said light source while applying light from successive picture elements to said single photocell for controlling the current flowing successively through each photoconductive element from said current source responsive to light variations.

2. Apparatus for converting the information on a document into an electrostatic charge pattern on a medium comprising means for depositing electrostatic charges on said medium responsive to light variations including a pluralty of photoconductive elements, a source of operating current for said photoconductive elements, means for controlling the amplitude of said operating current responsive to light variations including a single photocell connected to all of said photoconductive elements, a

source of light having a fixed level of illumination, and.

means for successively shining the light from said fixed level of illumination upon each of said photoconductive elements while shining successive picture elements from said document upon said single photocell for controlling the operating current responsive to light variations.

3. A system for reproducing the data on a document as an electrostatic charge pattern on a medium comprising a light to an electrostatic image transducer including I a plurality of separate photoconductive elements, a conductive bar spaced form said photoconductive elements, means for generating signals at a radio frequency, means for deriving video signals from the data on said document, and means for simultaneously applying said radio fre-' quency signals to said bar while applying said video signals across said plurality of photo conductive elements.

4. A system for reproducing the data on a document as an electrostatic charge pattern on a medium comprising a light to an electrostatic image transducer including a plurality of separate photoconductive elements arranged in a line, and a conductive barspaced from said line of photoconductive elements, said inedium passing between said line of photoconductive elements and said bar,means for applying signals at a radio frequency to said bar, a

photocell, a video signal amplifier connected to be driven by the output from said photocell, means for applying the output from said video signal amplifier between all of said photoconductive elements and said back bar, a source of illumination at a constant level, and means for suc- 5 6 cell with successive picture elements from said document. upon said photoconductive elements and on said photo- 5. Apparatus as recited in claim 4 wherein said light cell. source is positioned adjacent said document, and said References Cited by the Examiner means for successively illuminating said photoconductive UNITED STATES PATENTS elements from said light source while illuminating said 5 photocell with the successive picture elements from said 31205302 9/1965 Quade document comprises a many sided rotating mirror and means for positioning said mirror for reflecting light from DAVID REDINBAUGH Prlmary Examine" said light source and light from said document respectively H. W. BRITION, Assistant Examiner. 

1. IN A SYSTEM FOR CONVERTING A LIGHT IMAGE TO A CHARGE PATTERN ON A WRITING MEDIUM, EMPLOYING AN ELECTROSTATIC IMAGE TRANSDUCER OF THE TYPE COMPRISING A PLURALITY OF PHOTOCONDUCTIVE ELEMENTS EACH OF WHICH IS IN THE ELECTRICAL CURRENT PATH EMPLOYED FOR DEPOSITING ELECTROSTATIC CHARGES ON SAID MEDIUM REPRESENTATIVE OF LIGHT IMAGE VARIATIONS, THE IMPROVEMENT COMPRISING A SOURCE OF ELECTRICAL CURRENT, A SINGLE PHOTOCELL CONNECTING SAID SOURCE OF ELECTRICAL CURRENT TO ALL OF SAID PHOTOCONDUCTIVE ELEMENTS, A SOURCE OF LIGHT HAVING A FIXED LEVEL OF ILLUMINATION, AND MEANS FOR SUCCESSIVELY ILLUMINATION, AND ELEMENTS WITH LIGHT SAID LIGHT SOURCE WHILE APPLYING 